1. Field of the Invention
The present invention relates to arrangements for cooling voice coil motors, and in particular to arrangements for cooling voice coil motors for use in lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
patterning means, for patterning the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate.
2. Background of the Related Art
The terms xe2x80x9cpatterning meansxe2x80x9d xe2x80x9cmaskxe2x80x9d, or xe2x80x9cprojection beam patterning structurexe2x80x9d should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask table for holding a mask. The concept of a mask is well known in lithography, and its includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask table ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask table and mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple tablexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and U.S. Ser. No. 09/180,011 filed Feb. 27, 1998 (WO98/40791), incorporated herein by reference.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single substrate will contain a whole network of adjacent target portions which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once, such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a lithographic apparatus, voice coil motors (typically Lorentz-force motors) are used, for example, for fine positioning of the substrate table and the mask table. The motors dissipate heat in operation which, if not removed in a controlled manner, would affect the thermal stability of the apparatus. In the case of the fine positioning of a substrate table, it is important that heat transfer from the motor to the substrate table is minimized. The substrate table is designed to be operated at a predetermined temperature and heat transfer to the substrate table may lead to expansion or contraction in the substrate table and/or the substrate causing an inaccuracy in the positioning of the substrate and thus of the target portion to be irradiated.
At present the coils of the motors are cooled by a thin metal cooling plate in thermal contact with the coil. Heat from the coil is removed by thermal conduction through the cooling plate to its sides which are connected to a water cooling system. Thus there is inevitably a temperature variation across the cooling plate, and heat loss by convection or radiation from the surfaces of the cooling plate can lead to a heat load on the substrate table which is too large. For example, in a typical design no more than 0.5W out of approximately 350W of heat dissipated in each voice coil motor can be allowed to escape to the surrounding components without causing a detrimental effect.
An object of the present invention is to provide an improved means of cooling the voice coil motors of lithographic apparatus which will result in a reduction in the heat loss to its surroundings.
According to the present invention there is provided a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation;
patterning means, for patterning the projection beam according to a desired pattern;
a substrate table for holding a substrate;
a projection system for imaging the patterned beam onto a target portion of the substrate; wherein
at least one of the patterning means and the substrate table is associated with positioning means comprising at least one voice coil motor having a coil in thermal contact with a cooling jacket; characterized in that:
the cooling jacket comprises at least one channel for circulation of a cooling fluid, the or each channel being arranged such as to be substantially located in a portion of the cooling jacket adjacent to the coil.
The present invention is advantageous in that heat is removed directly from the coil to the cooling medium through a large area of contact, thus reducing substantially the amount of heat escaping to the surrounding components and therefore reducing inaccuracies caused by thermal expansion of the object table and any object thereon.
It is preferred that the cooling jacket is made from an electrically non-conducting material. This is advantageous as it will prevent undesirable eddy-current damping within the magnetic field of the stator. The use of a ceramic material, for example Al2O3 or AlN, is particularly desirable as the processing and properties of ceramic materials are already well known. Ceramic materials do not absorb water (which is a particular disadvantage of composite materials); they have high strength (and can therefore sustain a higher cooling fluid operating pressure for a given dimension) and they do not outgas (and are hence suitable for use in a vacuum).
In a preferred embodiment of the invention at least one part of a ceramic cooling jacket having at least one channel located therein, is formed as a single monolithic component. This is advantageous as this will provide the strongest possible structure for the cooling jacket. A monolithic component is less subject to the risk of delamination than laminated designs.
In another preferred embodiment of the invention at least one part of a ceramic cooling jacket, having at least one channel located therein is formed from two components which are bonded together to form said one part, at least one of the contacting surfaces of the components comprising at least one groove which forms the channel(s) in the bonded state. This is advantageous because it is then possible to provide a cooling jacket in which the thermal properties can be optimized for each position within the structure, rather than having uniform thermal properties across the cooling jacket which are a compromise.
In yet another preferred embodiment at least one part of a ceramic cooling jacket, having at least one channel located therein is formed from three components that are bonded together, an intermediate component being formed with sections of material removed, said sections forming the channels in the bonded state. This also advantageously allows the thermal properties of the separate parts of the structure to be optimized.
According to a further aspect of the invention there is provided a device manufacturing method comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning means to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, wherein at least one of said substrate table and said patterning means if positioned using at least one voice coil motor having a coil in thermal contact with a cooling jacket; characterized by: said step of passing a cooling fluid through the cooling jacket so as to substantially pass the cooling fluid through a portion of the cooling jacket adjacent to the coil.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-0672504.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet (UV) radiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), extreme ultraviolet (EUV) radiation, X-rays, electrons and ions. Also, in the present document reference may be made to X, Y, and Z axes, or to up, down and other similar spatial references. It should be understood that these terms are used with reference to the plane of the substrate and not relative to a general frame of reference. In the frame of reference used, the substantially planar substrate lies in a xe2x80x9chorizontalxe2x80x9d plane, defined by the X and Y axes. The Z axis denotes the xe2x80x9cverticalxe2x80x9d position and is perpendicular to the plane defined by the X and Y axes.